1. Field of the Invention
The present invention relates to an optical coupling method and an optical coupling device for optically coupling two optical waveguide devices differing in spot size of light wave.
2. Description of the Related Art
For example, when a semiconductor optical device such as a semiconductor laser diode and a semiconductor switch is optically coupled to an optical waveguide device such as a single mode optical fiber, even if an end of the semiconductor optical device is directly butted against and coupled to (butt-jointed to) an end of the optical fiber, a coupling loss is caused in the butting portion due to the difference in the spot sizes of the light waves of the optical waveguides.
Typically, the spot size (mode diameter) of a light wave emitted from a semiconductor optical device is about 1 .mu.m, while the spot size of a single mode optical fiber is about 5 .mu.m. When these devices are butt-jointed to each other, the coupling loss amounts to about 10 dB.
Heretofore, a method for reducing loss due to coupling a semiconductor optical device to an optical fiber has generally employed the use of a lens to cause the spot sizes to coincide with each other.
However, when the devices are optically coupled by the use of the lens or the like, assembly is difficult because tolerance is tight for aligning a lens, which disadvantageously raises the cost of the module including the lens. More particularly, recently the use of modules incorporating the semiconductor laser diode is rapidly spreading from basic transmission systems to subscriber systems, LAN (local area network) systems, data link systems and the like, and these systems require a large number of inexpensive modules.
As described above, since difficulty in the assembly process is the main cause of the high cost of a module incorporating the semiconductor laser diode, it is desirable that the assembly process be facilitated by reducing the number of components and by passive alignment. Thus, various optical coupling devices for optical coupling and a laser light source on which the optical coupling device and the semiconductor laser diode are integrated have been developed.
FIG. 1 shows an example of a laser light source on which a conventional optical coupling device and a semiconductor laser diode are integrated and the relationship of the coupling loss to the device length. The optical coupling devices introduced in cited references A-F shown in FIG. 1 are each provided with a tapered waveguide of hundreds of .mu.m in length for converting the spot size. For converting from one guided mode to another guided mode, these devices are adapted so that the width of the waveguide is gradually narrowed (tapered). so as to thereby reduce coupling loss.
As shown in FIGS. 2 and 3, the conventional optical coupling device comprises: substrate 13; first waveguide 11 which is a wide straight waveguide buried in substrate 13; and second waveguide 12 in which the end in contact with first waveguide 11 is the same width as first waveguide 11 and that width is gradually narrowed toward the other end of minimum width. In these drawings, L denotes the length of second waveguide 12, W1 denotes the width of the widest portion, W2 denotes the width of the narrowest portion, n1 denotes the refractive index of substrate 13, and nc (&gt;n1) denotes the refractive index of first waveguide 11 and second waveguide 12.
The above-described conventional optical coupling device has a problem in that an attempt to reduce the cost by shortening the device length increases the coupling loss as shown in FIG. 1.
In some laser light sources on which the optical coupling device and the semiconductor laser diode are integrated, an optical coupling portion is also used as an active layer (for example, M. Kito et al., OECC '96 Technical Digest, PP. 574-575, 1996). In this case, the longer the tapered waveguide is at the same device length, the more the gain of the semiconductor laser diode is reduced. This caises a problem in that the threshold current or operating current of a laser diode in a high-temperature environment is increased.
That is, in order to reduce the cost of the module and prevent the operating current from increasing, it is necessary to shorten the above mentioned optical coupling device and to reduce the coupling loss.